1. Field of the Invention
The present invention is a directed to methods and apparatus for providing power to devices on AC power lines. More particularly, the invention relates to methods and apparatus for providing power from AC lines to lighting devices. More particularly, the invention relates to methods and apparatus for providing power from AC lines controlled by dimmer circuits or electronics transformers to low-power lighting devices, such as LED lamps.
2. Description of Related Art
There are many lighting applications where amount of light is adjusted via a user-operated device, commonly known as a “dimmer”. Conventional AC dimmers reduce AC line voltage to a smaller amount by various methods. Commonly dimming is implemented by removing parts of the AC waveform. Perhaps the most commonly used dimmers, known as “leading-edge” dimmers, employ bidirectional triode thyristors or “triacs” to adjust turn-on time, producing an AC waveform with “chopped-off” leading parts. Triacs function as electronic switches which start to conduct when an appropriate voltage pulse is applied to its gate input, provided current that starts to flow through it is larger than a limit known as “latching current” (IL). Once a triac is conducting it will continue to conduct until current that flows through it drops below a current limit known as “holding current” (IH). Once that happens, the triac will stop conducting.
This technique requires a minimal load that will require currents larger than the latching and holding currents. Many commercially available and installed dimmers of this type require at least a 15 W load on a 110VAC line to work properly. If the load drops below that level the dimmer will not provide smooth dimming, but will instead exhibit such behavior as flickering, audible buzzing, abrupt intensity changes and reduced dimming range.
Triac dimmers are designed to be used with incandescent light bulbs, where 20 W is generally the minimal expected load.
LED lighting is an emerging energy-efficient illumination method. A primary advantage of LED lamps is their power efficiency—the usual LED lamp consumes 3 W to 6 W, with the majority of them consuming less than 10 W. This power consumption is below the 15 W required for normal operation of a triac-based dimmer. It is very desirable to be able to use new LED lamps in existing installations that are controlled by triac dimmers.
Another similar application is the replacement of halogen lamps with LED lamps. Halogen lamps usually require 12V to operate—which could be 12VDC or 12VAC. A common power source is 12VAC generated from the mains by means of a device known as an “electronic transformer”. This device generates 12VAC at a frequency in the range of 20 kHz-60 kHz. Electronic transformers usually require 10 W-30 W minimum load to operate properly. LED lamps are an excellent energy-efficient substitute for such halogen lamps but their power is usually less than the minimum required by an electronic transformer.
Common methods of dealing with these problems involve requiring usage of more than a single LED lamp to increase total power load, or to increase power consumption of the LED driver, wasting energy and generating extra heat. There is great interest in creating drop-in low-power LED lamp replacements which could be used instead of incandescent or halogen light bulbs in a manner completely transparent to the user.
FIG. 1 is a block-diagram of a conventional method of driving an incandescent light bulb using triac dimmer 10. Input sinusoidal AC voltage 16 (shown on FIG. 3) is charging capacitor 13 through variable resistor 12. After voltage across capacitor 13 exceeds breakdown voltage of diac 14, a pulse will be generated which will be applied to gate of triac 11. This in turn will turn triac 11 on and it will start conducting current, which will flow through incandescent light bulb 20. If that current exceeds a latching current limit it will continue to flow for the rest of a half-period of the AC waveform until it drops below holding current levels. At the same time capacitor 13 will discharge through diac 14 and triac 11 and will be ready to generate another pulse on the next half-period of the AC waveform.
The moment when a gate pulse is generated is defined by the resistance of variable resistor 12 and the capacitance of capacitor 13. Users can change that time, known as phase angle, by adjusting resistance of variable resistor 12.
FIG. 3 illustrates the 110V AC waveform at point 16 (on the input of dimmer 10), common in the USA.
FIG. 4 illustrates one possible waveform on the output of dimmer 10 at point 15, when resistance of variable resistor 12 is set to minimum. The beginning of each half-period is removed because setting the phase angle to start earlier might cause firing when current through triac 11 is less than latching current. This is the same waveform that is being applied to light bulb 20 through wires 17.
FIG. 5 illustrates another possible waveform with phase angle set to approximately 70 degrees.
FIG. 6 illustrates another possible waveform with minimum phase angle when the load is too small for triac 11 to work properly. Even though phase angle is set to minimum, the triac behaves like its phase angle is set to approximately 45 degrees.
FIG. 7 illustrates another possible waveform with phase angle set to 70 degrees, when the load is too small for triac 11 to work properly. In this situation triac 11 is not turning on at all.
FIG. 2 shows one prior art method of driving an LED lamp with a dimmer.
The AC output 41 of dimmer 10 is fed through wires 40 to full-wave rectifier 31. The rectified signal 42 from rectifier 31 is filtered by inductor 33 and capacitor 34. That signal 42 provides power to LED driver 32, which in turn generates voltage to drive LED light 50, which can be a single LED or a plurality of high-brightness LEDs. In order to maintain current through the triac 11 in dimmer 10 above latching and holding levels, bleeder resistors 35 and 37 are employed, together with switch 36.
LED driver 32, using the dimmer voltage 41, measured through path 39, to maintain necessary holding current, will temporary add load resistor 35 through switch 36 controlled by signal 38 when required. Bleeder resistors 35 and 37 are used every half-waveform and they increase power consumption of LED light 50 to levels where dimmer 10 can operate normally. This method artificially lowers efficiency of LED light system by increasing its power consumption, and thereby destroys one of the major advantages LED light has over incandescent lamps.